American Plate Motion Research Articles

Paleomagnetic Rotations in the Northeastern Caribbean Region Reveal Major Intraplate Deformation Since the Eocene

AbstractRelative Caribbean‐North American plate motion is partitioned over the trench and intra‐Caribbean plate faults that bound large scale tectonic blocks. Quantifying the kinematic evolution of this tectonic corridor is challenging because much of the region is submarine. We present an extensive regional paleomagnetic data set (1,330 cores from 136 sampling locations) from Eocene and younger rocks of the northern Lesser Antilles, the Virgin Islands, and Puerto Rico, and use a statistical bootstrapping approach to quantify vertical axis block rotations. Our results show that the Puerto Rico–Virgin Island (PRVI) block and the Northern Lesser Antilles (NoLA) block formed two coherently rotating domains that both underwent at least 45° counterclockwise rotation since the Eocene. The first ∼20° occurred in tandem in late Eocene and Oligocene time, after which the blocks were separated in the Miocene by the opening of the Anegada Passage. The last 25° of rotation of the PRVI block ended in the middle Miocene, whereas the NoLA block rotated slower, until the latest Miocene. The boundary between the NoLA block and a non‐rotated Southern Lesser Antilles was likely the Monserrat‐Harvers fault zone. These results require hundreds of kilometers of intra‐Caribbean motions with oroclinal bending of the trench or forearc sliver motion along the curved plate boundary as endmembers. These data invite a critical re‐evaluation of the kinematic reconstruction of Caribbean‐North American plate motion. The consequent changes in paleogeography may provide a new view on the enigmatic eastern Caribbean paleo‐biogeography and the Paleogene dispersal of South American mammals toward the Greater Antilles.

Contemporary deformation and stressing rates in Southern Alaska

SUMMARY The subduction of the Pacific Plate beneath North America induces broad scale stressing of the Alaskan crust that has led to the development of the highest mountains in North America, the highest slip rates along some of the longest strike-slip faults on Earth, and widespread seismicity that includes the 1964 M9.2 Alaska earthquake, the second largest ever recorded. These features are a consequence of deformation associated with three primary processes, interseismic loading due to relative plate motions, large earthquakes and postseismic processes. How these mechanisms contribute to the evolution of stress in the Alaskan crust is not well understood. Here we use observed contemporary surface velocities to constrain 2-D and 3-D viscoelastic numerical models of relative Pacific/North American plate motions, coseismic slip associated with the 1964 (M9.2) megathrust event and strike-slip earthquakes on the transform boundary in 1949 (M8.1), 1958 (M7.8) and 1972 (M7.6) (the four largest events prior to the 2002 M7.9 Denali quake), viscoelastic relaxation following these events, and afterslip, to gain insight into how these processes are shaping Alaska today. Results suggest that interseismic deformation and on-going post-seismic deformation following the 1964 earthquake both contribute significantly to the GPS measured contemporary velocity field. Viscoelastic relaxation associated with a mantle with a viscosity of � 10 19 Pa s is required to explain southerly directed velocities that are observed in the Cook Inlet region to well north of the Denali fault. Results also suggest that subduction of the Pacific Plate leads to a broad zone of deformation with high stressing rates concentrated in a band that lies several hundred kilometres from the plate boundary, coeval with the inboard location of the maximum locking depth of the megathrust. Interseismic deformation and stressing rates remain high further inland across the Yakutat microplate, where flat subduction extends the width of the locked plate interface. Calculations show that post-seismic relaxation following the large strike-slip events serves to reload these rupture surfaces while relieving stress on the eastern Denali Fault. Post-seismic relaxation following the 1964 earthquake combined with coseismic stress changes, promoted the triggering of the 2002 Denali quake. Calculations also suggest that over the past 50 years high stress has accumulated on part of the thrust interface to the west and east of the 1964 rupture surface and along the Queen-Charlotte Fault to the south of the 1949 rupture surface.

Fault type analysis along the San Andreas Fault zone: A numerical approach

Finite Element (FE) modeling under plane stress condition is used to analyze the fault type variation with depth along and around the San Andreas Fault (SAF) zone. In this simulation elastic rheology was used and was thought justifiable as the variation in depth from 0.5 km to 20 km was considered. Series of calculations were performed with the variation in domain properties. Three types of models were created based on simple geological map of California, namely, 1) single domain model considering whole California as one homogeneous domain, 2) three domains model including the North American plate, Pacific plate, and SAF zone as separate domains, and 3) Four domains model including the three above plus the Garlock Fault zone. Mohr-Coulomb failure criterion and Byerlee’s law were used for the calculation of failure state. All the models were driven by displacement boundary condition imposing the fixed North American plate and Pacific plate motion along N34°W vector up to the northern terminus of SAF and N50°E vector motion for the subducting the Gorda and Juan de Fuca plates. Our simulated results revealed that as the depth increased, the fault types were generally normal, and at shallow depth greater strike slip and some thrust faults were formed. It is concluded that SAF may be terminated as normal fault at depth although the surface expression is clearly strike slip.

Tectonic and Kinematic Regime along the Northern Caribbean Plate Boundary: New Insights from Broad-band Modeling of the May 25, 1992, M s = 6.9 Cabo Cruz, Cuba, Earthquake

—On May 25th, 1992, an M s = 6.9 earthquake occurred off the southwestern tip of Cuba, along the boundary between the Caribbean and North American plates. This earthquake was the largest to strike southern Cuba since 1917 and the largest ever recorded in that region by global seismic networks. It is therefore a key element for our understanding of the tectonic and kinematic regime along the northern Caribbean plate boundary. In order to test the previously proposed source parameters of the Cabo Cruz earthquake and to better constrain its focal mechanism, we derived a new set of source parameters from unfiltered broad-band teleseismic records. We used a hybrid ray tracing method that allows us to take into account propagation effects of seismic waves in a realistic crustal model around the source. Our solution is consistent with the long-period focal mechanism solution of Virieux et al. (1992). Our solution also models the higher frequency crustal and water layer phases. The primarily strike-slip focal mechanism has a small thrust component. Its shows an east-west trending nodal plane dipping 55° to the north that we interpret as the rupture plane since it corresponds to the geometry of the major active fault in that area. The displacement on this plane is a left-lateral strike-slip combined with a small amount of southward thrust. The result is in good agreement with the active tectonic structures observed along the Oriente fault south of Cuba. The small thrust component demonstrates that, contrary to prior belief, the transpressive regime extends along this whole segment of the Caribbean/North American plate boundary. Together with historical seismicity, it suggests that most of the stress accumulated by the Caribbean/North American plate motion is released seismically along the southern Cuban margin during relatively few but large earthquakes.

Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone

From the aftershocks of the 1992 Landers earthquake, we infer the orientation of the principal strain rate axes (d1 > d2 > d3; d1 lengthening), their relative magnitude, and the relative spin of fault blocks by using a micropolar continuum model to invert the seismic P and T axes. The seismogenic deformation is consistent with the geodetic measurements of the coseismic displacement and with the secular deformation of the central Mojave block. Regionally, the aftershock data define two major domains within the central Mojave block: (1) the western Mojave block, including the San Bernardino Mountains and the epicentral area of the Big Bear earthquake, which is characterized by E‐W d1 (lengthening) and N‐S d3 (shortening); and (2) the central Mojave block, including the Landers surface rupture zone, which is characterized by NW‐SE d1 and NE‐SW d3. Inversion for the principal strain axes of geodetically measured coseismic displacements across the Big Bear and Landers seismogenic zones gives results similar to the aftershock inversions for those areas, indicating that the aftershocks accommodate a deformation similar to the main rupture and do not reflect elastic rebound or residual stresses. The background seismicity for 1981 to 1991 shows the same characteristic d1 and d3 orientations for the two domains, indicating that the secular seismogenic strain has the same regional geometry as the 1992 coseismic deformation. The micropolar inversion also provides values of the relative vorticity parameter W, which reflects a difference between the vorticity of a shearing continuum and the vorticity of fault‐bounded blocks rotating within tabular seismogenic shear zones. The observed fault geometry along the Kickapoo fault suggests a pinned‐block model for the local block rotation that is consistent with the values of W obtained from our inversions. We interpret the regional NW‐SE orientation of d1 in the central Mojave block to be characteristic of the dextral eastern California shear zone, which transfers approximately 22% of the Pacific‐North American plate motion from the San Andreas system to the Walker Lane Belt in eastern California. Our results and geodetic determinations of the secular shear strain in the central Mojave block indicate that the locus of NW dextral shear generally lies between the San Bernardino Mountains and the Pisgah fault.

Variation of seismic slip in the Gulf of California and the possible effect on geodetic measurements of Pacific‐North American Plate motion

The Gulf of California is a unique tectonic transition zone which progresses from an oceanic ridge‐transform system at the mouth of the gulf to a predominantly continental transform system in southern California. Slip rates estimated from seismic moment data amount to about 17 to 30% of the accumulated tectonic slip in the gulf predicted by the NUVEL‐1 plate motion model, even after accounting for the transition in crustal structure. The seismic to tectonic slip ratios also appear to increase slightly from south to north. This trend in the seismic moment release may reflect changes in the accommodation of Pacific‐North American plate motion along the gulf which can be examined geodetically. On the other hand, geodetic measurements of the relative plate motion can be disturbed by short‐term effects associated with episodic seismicity. To assess these effects, the surface displacements due to typical transform events in the gulf are estimated using a simple dislocation model. The results of this numerical calculation suggest that if a large transform event (M0 ∼ 1.5 × 1026 dyne cm) were to occur within 100 to 200 km of a geodetic baseline, for example one comprised in the GEOMEX Global Positioning System network across the southern gulf, the relative distance measurements could be affected by up to 15 mm. This is marginally at the error level of a few millimeters plus 2 parts in 108 of baseline length for GEOMEX measurements, which thus are sensitive only to the far‐field displacement along the plate boundary (over baselines of a few hundred kilometers in length).

Formation of interior basins associated with curved faults in Alaska

The positions of certain nonmarine sedimentary basins in central and southern Alaska are spatially related to the geometry of adjacent curved faults. These interior basins occur along the northern, convex side of these fault systems which change trend from northwest in Canada and eastern Alaska to east‐west in central Alaska and to northeast in western Alaska. The basins probably initiated in early to middle Tertiary time and deposition within all basins continued through the Quaternary, broadly synchronous with Tertiary strike‐slip displacements along the Tintina, Denali, and Border Ranges fault systems. We examine the spatial and temporal relationship between basins and faults by using a mechanical model of curved strike‐slip faults based on boundary elements. We relate slip along the curved faults to Tertiary Pacific‐North American plate motions by using principal stress directions inferred from the plate motion directions. Locations of the predicted mean stress reductions around the fault bends caused by fault slip correlate well with positions of the interior basins. Our results suggest that basin subsidence and location were controlled primarily by the geometry of adjacent curved fault systems and the state of stress in Alaska during the Tertiary. Mechanical interaction between curved Alaskan fault systems increases the tendency for basin subsidence. Stresses inferred from early Tertiary plate motion vectors are not oriented correctly to drive right lateral slip along Kaltag, Farewell, and parallel faults in western Alaska. Furthermore, these stress states would have produced left lateral displacements, for which there is no evidence, along these western faults in their present orientations. We find that a combination of slightly rotated fault geometry and low friction along the faults appears to facilitate right lateral slip along the faults in western Alaska. Our results show that deformation well inside the continent can be explained in terms of the Pacific‐North American plate motions. Thus Tertiary plate motions have resulted in a broad zone of fault slip, uplift, and subsidence in the nonrigid North American plate, producing several large basins.

Role of the Eastern California Shear Zone in accommodating Pacific‐North American Plate motion

The newly recognized Eastern California shear zone (ECSZ) of the Mojave Desert‐Death Valley region has played a major, but previously underappreciated role in accommodating the dextral shear between the Pacific and North American plates in late Cenozoic time. Comparison of integrated net slip along the shear zone with motion values across the entire transform boundary indicates that between 9% and 23% of the total relative plate motion has occurred along the ECSZ since its probable inception ∼10–6 Ma. Long‐term integrated shear along the ECSZ (6–12 mm yr−1) is similar to historic measurements (6.7±1.3 mm yr−1). Time‐space patterns of faulting suggest that shear was concentrated in the eastern part of the Mojave Desert block and Death Valley during late Miocene and early Pleistocene time, but that the locus of faulting in the south‐central Mojave jumped westward between 1.5 and 0.7 Ma.

Energy constraints on kinematic models of oblique faulting: Loma Prieta versus Parkfield‐Coalinga

The Loma Prieta earthquake was a combination of right lateral strike‐slip and reverse faulting. This oblique motion has been explained by a kinematic model that has the fault accommodate both the translation and convergence required by the Pacific‐North American plate motions and a local bend in the San Andreas Fault However, in other areas of California, such as Parkfield‐Coalinga, this combination of motions is taken up by decoupled strike‐slip and thrust faults. In this paper I develop a simple physical model, based on applying Hamilton's principle to the energy released in earthquakes, that allows us to determine if a region should undergo oblique or decoupled faulting. This determination depends on the dip of available faults and the amount of convergence relative to the amount of translation. These energy constraints correctly predict the different types of faulting at Loma Prieta and Parkfield‐Coalinga.

Motion of Caribbean Plate during last 7 million years and implications for earlier Cenozoic movements

The direction and rate of movement of the Caribbean plate with respect to North America are determined from the slip vectors of shallow earthquakes and from the configuration of downgoing seismic zones in the Greater and Lesser Antilles. A calibration of the relative plate motion for the northeastern Caribbean using data from other subduction zones indicates an average rate of 3.7±0.5 cm/yr for the past 7 million years (Ma). The direction of plate motion inferred from focal mechanisms (ENE) is nearly the same as that deduced from the configuration of downgoing seismic zones going around the major bend in the arc. With respect to North America, the Caribbean plate is moving at an angular velocity of 0.36°/Ma about a center of rotation near 66°N, 132°W. Vector addition using those data and that for the relative motion of North and South America indicates that the Caribbean is moving at an angular velocity of 0.47°/Ma about a center of rotation near 60°N, 88°W with respect to South America. The presence of intermediate‐depth earthquakes beneath Puerto Rico and the Virgin Islands is ascribed to the curvature of the plate boundary and a component of underthrusting that has been going on for at least the past 7 Ma and is likely occurring today. The alternative hypothesis that earthquakes beneath those areas are occurring in materials that were subducted during the Eocene, the last major episode of magmatism, is not tenable from thermal considerations. The lack of recent magmatism in the eastern Greater Antilles is ascribed to the relatively small component of underthrusting. The 2 cm/yr rate of seafloor creation along the mid‐Cayman spreading center for the past 2.4 Ma does not appear to reflect the total Caribbean‐North American plate motion while the 4 cm/yr spreading rate from 6.0 to 2.4 Ma does. Between the mid‐Cayman spreading center and eastern Guatemala, the northern boundary of the Caribbean plate is narrow and follows the southern margin of the Cayman trough. Seismic activity between the spreading center and eastern Hispaniola, however, occurs over a zone about 250 km wide that extends from Cuba to Jamaica and across the entire width of Hispaniola. Individual faults within this broad plate boundary appear to have accommodated differing amounts of motion as a function of geological time while the cumulative plate motion across the zone remained nearly constant. The percentage of total plate motion accommodated near southern Hispaniola and Jamaica is inferred to have increased about 2.4 Ma ago. That change may have been caused by the collision of parts of the Bahama bank and northern Hispaniola. This explanation for the sudden decrease in seafloor creation along the mid‐Cayman spreading center is less catastrophist than the hypothesis that the entire Caribbean plate suddenly changed its velocity with respect to surrounding plates. The Caribbean plate may be regarded as a small buffer plate whose motion is now governed by the movement of the larger North and South American plates which bound it on three sides. The Caribbean plate is either at rest or moving eastward at a rate of no more than 1 cm/yr in the hot spot reference frame. Since the relative motion of the larger plates surrounding the Caribbean has been nearly constant for the last 38 Ma (anomaly 13 time) and since the forces on the Caribbean plate do not appear to have changed greatly during that interval, we extrapolate the motion of the last 7 Ma back to 38 Ma. A reconstruction for the late Eocene places the Caribbean plate about 1400 km west of its present position. The faster rate of plate motion we calculate makes it more likely that the lithosphere beneath the basins of the Caribbean originated in the Pacific. It also has implications for the seismic potential of the region, paleocirculation in the Atlantic Ocean and origin of sediments in the area. Our late Eocene reconstruction aligns the eastern continental margin of Yucatan with that along the southeast side of the Nicaragua rise. This 2500‐km‐long feature may have acted as an arc‐arc transform fault from the late Mesozoic to the late Eocene. Arc‐related rocks of those ages in the Greater Antilles and northern Lesser Antilles define a northwesterly trending subduction zone along the northeastern edge of the former East Pacific‐Caribbean plate. At least three fragments of anomalous seafloor have been sutured onto Hispaniola in the past 50 Ma.